Open magnet device and magnetic resonance imaging apparatus comprising it

ABSTRACT

In a superconductive magnet device comprising upper and lower vacuum vessels coupled by a coupling conduit, the upper and lower vacuum vessels  9  and the coupling conduit  24  coupling them are made of a high electric-resistance material so that the magnetic flux of gradient magnetic field coils reaches the upper and lower pair of a second heat shields  8  and a coupling conduit  23  coupling the pair of the second heat shields  8.  The coupling conduit  23  coupling the upper and lower pair of the second heat shields  8  is provided with a slit (cut)  41  extending in the vertical direction so that no eddy currents occur and thus eddy currents generated asymmetrically in the coupling conduit  23  are reduced. Artifacts caused by asymmetrical eddy currents can by reduced in an MRI apparatus using such a magnet device.

TECHNICAL FIELD

The present invention relates to a magnet device used for a magneticresonance imaging apparatus (hereinafter referred as “MRI apparatus”).In particular, it relates to a technique for suppressing eddy currentsproduced within the magnet device by gradient magnetic field coils of anMRI apparatus.

RELATED ART

A conventional MRI apparatus is equipped with a static magnetic fieldmagnet for generating a static magnetic field, gradient magnetic fieldcoils for generating gradient magnetic fields and an RF coil, andapplies RF pulses produced by the RF coil to a subject under examination(e.g., a patient) placed in the static and gradient magnetic fields toexcite atomic nuclear spins, and detects NMR signals emitted by thespins to produce tomograms of the subject.

Such MRI apparatuses are often equipped with a superconductive magnet asthe static magnetic field magnet. While a superconductive magnet istypically a solenoid type magnet that generates a magnetic field in thedirection of the body axis of the patient, an open type has been alsodeveloped in recent years, which facilitates interventional MRI (IVMR)using an MRI apparatus for monitoring surgical operations and so forth,and relieves the subject (patient) from a sense of confinement. In theopen type magnet, a pair of superconductive magnets is disposedvertically so as to define a space where the patient is placed, and theupper and lower superconductive magnets are housed in vacuum vesselsinterconnected by coupling conduits. In this type of superconductivemagnet device, the gradient magnetic field coils and RF coil aredisposed in the vicinity of the vacuum vessels including the upper andlower superconductive magnets.

In such a conventional superconductive magnet device, materials of lowelectric resistance such as aluminum alloy have been used as thematerial for the vacuum vessels and the coupling conduits for connectingthe vacuum vessels, in view of considerations such as strength, sealingproperty and cost. As a result, eddy currents are generated in thevacuum vessels and coupling conduits upon operation of the gradientmagnetic field coils. Further, the vacuum vessels and coupling conduitsinclude heat-shield plates that are kept at an extremely low temperaturesuch as 20K or 80K in order to intercept exterior heat. Since such aheat shield plate is made of a material of good heat conductivity andlow electric resistance, the problem of generation of eddy currents alsoarises in these plates as in the vacuum vessels.

In some superconductive magnets, as shown in FIG. 9, in which the z axislies in the vertical direction and the x and y axes lie in thedirections orthogonal to the vertical direction, coupling conduits 24vertically connecting the vacuum vessels 9 are installed asymmetricallywith respect to the x axis in order to facilitate access to ameasurement space. In superconductive magnets of this type, eddycurrents generated in the coupling conduit portions 24 due to changes inmagnetic fields generated by the gradient magnetic field coils areasymmetrical with respect to the x-axis (in the y direction). Theasymmetrical eddy currents cause artifacts in images produced by the MRIapparatus and degrades image quality. Such asymmetrical eddy currentscannot be corrected by adjusting the current waveform of the magneticfield generating unit.

Therefore, an object of the present invention is to reduce degradationof images due to the asymmetrical eddy currents generated in a magnetdevice of an MRI apparatus, especially in a superconductive magnet.

DISCLOSURE OF THE INVENTION

In order to attain the aforementioned object, a magnet device of thepresent invention comprises:

a pair of vacuum vessels disposed across a space in a vertical directionthat is an arbitrarily defined direction,

cylindrical vacuum vessel coupling means for vertically coupling theinner spaces of said pair of vacuum vessels, and

a pair of magnetic field generating means each of which is disposed inone of the pair of the vacuum vessels and includes a material havingsuperconductivity,

wherein the pair of vacuum vessels and the vacuum vessel coupling meansare made of a material having a high electric resistance.

In one embodiment of the magnet device according to the presentinvention, in which the pair of vacuum vessels and the vacuum vesselcoupling means are made of a high electric-resistance material asaforementioned, the magnet device further comprises at least one pair ofheat shields disposed inside the pair of vacuum vessels and cylindricalheat shield coupling means for vertically coupling the pair of heatshields through the space surrounded by the vacuum vessel couplingmeans, wherein the heat shield coupling means have slits exhibiting ahigh electric-resistance property and extending continuously orintermittently in the vertical direction. The term “exhibiting a highelectric-resistance property” includes the case where a highelectric-resistance is exhibited due to an air gap at the slit portionand the case where a high electric-resistance is exhibited due to a highelectric-resistance material covering the slit portion.

According to this embodiment, generation of asymmetric eddy currents inthe vacuum vessels and vacuum vessel coupling means can be prevented.Particularly in the case that the heat shield coupling means havecontinuous or intermittent slits, no current that would be a cause forgeneration of asymmetric eddy currents in the heat shields flows in theheat shield coupling means. The slit portion is preferably positioned atleast on the center side of the heat shield coupling means in theanterior and posterior direction and in the transverse (right and left)direction (that is, on the measurement space side).

In a second embodiment of the magnet device according to the presentinvention, in which the pair of vacuum vessels and the vacuum vesselcoupling means are made of a high electric-resistance material asaforementioned, the magnet device further comprises at least one pair ofheat shields disposed inside the pair of vacuum vessels and cylindricalheat shield coupling means for vertically connecting the heat shieldsthrough the space surrounded by the vacuum vessel coupling means,wherein the heat shields have holes opening into the inner space of theheat shield coupling means and recesses having the same shape as thoseof the holes in positions symmetrical to the holes.

By symmetric positions is meant that the positions are symmetrical withrespect to the center axis in the anterior and posterior direction andwith respect to the center axis in the transverse (right and left)direction of the heat shields. Provision of the recesses in suchpositions, which are electrically equivalent to the holes, reducesasymmetry of eddy currents caused by presence of the holes.

According to the first and second embodiments of the present invention,the problem of asymmetric eddy currents can be solved not on the vacuumvessel side, which is required to meet a severer condition of highsealing, but on the heat shield side.

In the magnet device of a third embodiment of the present invention, themagnet device comprises: a pair of vacuum vessels disposed across aspace in a vertical direction that is an arbitrarily defined direction,cylindrical vacuum vessel coupling means for vertically coupling theinner spaces of said pair of vacuum vessels, and a pair of magneticfield generating means each of which is disposed in one of the pair ofthe vacuum vessels and includes a material having superconductivity,wherein the vacuum vessel coupling means have slit portions extendingcontinuously or intermittently in the vertical direction and members forcovering the slit portions to maintain the vacuum vessels in a vacuumstate.

In a fourth embodiment of the magnet device according to the presentinvention, the magnet device comprises: a pair of vacuum vesselsdisposed across a space in a vertical direction that is an arbitrarilydirection, cylindrical vacuum vessel coupling means for verticallyconnecting the inner spaces of said pair of vacuum vessels, and a pairof magnetic field generating means each of which is disposed in one ofthe pair of the vacuum vessels and includes a material havingsuperconductivity, wherein the vacuum vessels have holes opening intothe inner space of the vacuum vessel coupling means and recesses havingthe same shape as those of the holes in positions symmetrical to theholes. Here too, “symmetrical position” means that the positions aresymmetrical with respect to the center axis in the anterior andposterior direction and with respect to the center axis in thetransverse direction of the vacuum vessel.

According to the third and fourth embodiments, the generation ofasymmetric eddy currents in the vacuum vessels and the coupling meansthereof is prevented. In this case too, the slit portion of the vacuumvessel coupling means is preferably provided at least on the center sidein the anterior and posterior direction and in the transverse directionof the heat shields (that is, on the measurement space side).

In a fifth embodiment of the magnet device according to the presentinvention, the magnet device has a pair of vacuum vessels and vacuumvessel coupling means made of a high electric-resistance material asaforementioned, and further comprises at least one pair of heat shieldsdisposed inside the pair of vacuum vessels and cylindrical heat shieldcoupling means for vertically connecting spaces surrounded by the vacuumvessel coupling means, wherein the heat shield coupling means have slitportions exhibiting a high electric resistance and extendingcontinuously or intermittently in the vertical direction, and the heatshields have holes opening into the inner space of the heat shieldcoupling means and recesses having the same shape as those of the holesin positions symmetrical to the holes.

Further, a magnet device of the present invention comprises: a pair ofmagnets disposed across a space in a vertical direction that is anarbitrarily defined direction for generating a static magnetic field,and supporting columns for coupling the magnets, wherein the columnshave slit portions extending continuously or intermittently in thevertical direction.

In this magnet device, the columns may be hollow. The magnets areprovided with covers for covering the magnets and the covers have holesopening into the inner space of the columns and recesses having the sameshape as those of the holes in positions symmetrical to the holes.

An MRI apparatus of the present invention comprises the aforementionedmagnet device as static magnetic field generating means. Typically, theMRI apparatus of the present invention comprises static magnetic fieldgenerating means for generating a static magnetic field in a space wherea subject (e.g., patient) is placed, gradient magnetic field generatingmeans for generating gradient magnetic fields in the space, atransmitting unit for applying an RF magnetic field to the subject, adetecting unit for detecting NMR signals emitted from the subject, asignal processor for processing the NMR signals to produce images orspectra of the subject, wherein the static magnetic field generatingmeans is a magnet device according to the present invention.

According to the present invention, asymmetric eddy currents can bereduced without need for a special correcting coil or other means forcorrecting asymmetric eddy currents or means for reducing leakage ofmagnetic flux from the gradient magnetic field coil. Since generation ofspatially asymmetric eddy currents in the magnet device can be thusprevented effectively, deterioration of images due to eddy currents canbe minimized in the MRI apparatus employing the magnet device accordingto the present invention.

BRIEF EXPLANATION OF DRAWINGS

FIG. 1 is a perspective view of a superconductive magnet deviceaccording to the present invention.

FIG. 2 is a cross-sectional view of the superconductive magnet device ofFIG. 1 sectioned along a plane perpendicular to the z direction.

FIG. 3 is a cross-sectional view of the superconductive magnet device ofFIG. 1 sectioned along a plane parallel to the z direction.

FIG. 4 is a perspective view of the lower portion of a vacuum vesselaccording to a first embodiment of the present invention.

FIG. 5 is a perspective view of the lower portion of the interior of avacuum vessel according to the first embodiment of the presentinvention.

FIG. 6 is a perspective view of the lower portion of the interior of avacuum vessel according to a second embodiment of the present invention.

FIG. 7 is a perspective view of the lower portion of the interior of avacuum vessel according to a third embodiment of the present invention.

FIG. 8 is a cross-sectional view of hole and aluminum lid structuresaccording to the third embodiment of the present invention.

FIG. 9 shows generation of eddy currents in the conventionalsuperconductive magnet.

FIG. 10 is an overall view of an MRI apparatus to which the magnetdevice of the present invention is applied.

PREFERRED EMBODIMENTS OF THE PRESENT INVENTION

Preferred embodiments of the present invention will now be explained.

The general structure of a superconductive magnet device of an MRIapparatus to which the present invention can be applied, and theconfiguration of the magnet device, gradient magnetic field coils and RFcoils will be explained first.

FIG. 1 is an overall perspective view of a superconductive magnet. FIG.2 is a cross-sectional view of the superconductive magnet of FIG. 1sectioned along a plane perpendicular to the z direction, and FIG. 3 isa cross-sectional view of the superconductive magnet sectioned along aplane parallel to the z direction of FIG. 1 and shows the configurationof gradient magnetic field coils and RF coils.

As shown in FIG. 1, the superconductive magnet unit has a structurewherein a pair of vacuum vessels 9, each of which includes asuperconductive magnet, are disposed face to face so as to sandwich acertain space 5 (a measurement space where a subject is placed in theMRI apparatus). The upper and lower vacuum vessels 9 are interconnectedby coupling conduits 24. The top and bottom of the whole assembly aresurrounded by iron plates 33 and the upper and lower iron plates 33 arecoupled magnetically by iron columns 32.

As shown in FIG. 1, the coupling conduits 24 of the vacuum vessels 9 andthe iron columns 32 are disposed posterior the measurement space 5(backside) with respect to the Y direction. This arrangement enables anoperator to freely access a patient inserted into the measurement space5 from the side and facilitates various treatments including IVMR. Apatient situated in the measurement space 5 of the superconductivemagnet unit has a broad horizontal field of view and feels lessconstricted and easier about being examined.

As shown in FIG. 3 in detail, inside the pair of vacuum vessels 9, thereare provided a pair of upper and lower superconductive coils 2, a pairof upper and lower helium containers 6, a pair of upper and lower firstheat shields 7, and a pair of upper and lower second heat shields 8.

Each of the upper and lower superconductive coils 2 is enclosed andsupported in the helium container 6 as immersed in liquid helium. Theupper and lower helium containers 6 are interconnected across aprescribed distance by coupling conduits 21. Each of the upper and lowerhelium containers 6 is surrounded by the first heat shield 7 and thesecond heat shield 8 located outside the first one. The couplingconduits 21 are surrounded by coupling conduits 22, 23, whichrespectively connect the upper and lower first heat shields and theupper and lower second heat shields across prescribed distances.

The upper and lower superconductive coils 2, helium containers 6, thefirst heat shields 7 and the second heat shields 8 are respectivelycontained in the upper and lower vacuum vessels 9. The coupling conduits21 and coupling conduits 22, 23 are contained in the coupling conduits24 which connect the upper and lower vacuum vessels 9 across aprescribed distance. The upper and lower vacuum vessels 9 arerespectively fixed to the upper and lower iron plates 33 by bolts 30 atfour positions. In addition, as shown in FIG. 3, the MRI apparatus usingthe superconductive magnet unit is further provided with gradientmagnetic field coils 50 and RF coils 51 between the upper and lowervacuum vessels 9 so as to sandwich the measurement space 5.

In such a structure, the superconductive coils 2 generate a uniform andstrong magnetic field in the measurement space 5 in the verticaldirection. The superconductive coils 2 are immersed in liquid helium inthe helium containers 6 and thereby kept cool and in a superconductivestate.

The helium containers 6 are insulated from the exterior heat by thevacuum of the vacuum vessels 9, and cooled by the first heat shields 7and the second heat shields 8 disposed outside the first heat shields,which are cooled to an extremely low temperature by a refrigerator (notshown in figures). Thus, evaporation of helium in the helium containerscan be suppressed. Further, the coupling conduits 22, 23 in thisstructure are made of a highly heat-conductive material. Thus, arefrigerator is need not be provided for each of the upper and lowervacuum vessels 9. The first and second heat shields 7 and 8 are made ofa material having excellent heat-conductivity and accordingly lowelectric resistance.

Magnetic shielding by the iron plates 33 is achieved by surrounding theupper and lower sides of the superconductive coils 2 with aferromagnetic material and by forming a magnetic path (return path) formagnetic flux generated by the superconductive coils 2, thereby reducingleakage of the magnetic field and keeping it from spreading far away.While a material other than iron can be employed for the magnetic shieldso far as it has ferromagnetic property, iron is generally preferredfrom the viewpoint of magnetic properties, cost and mechanical strength.

The magnet device of the present invention having the aforementionedbasic structure is further provided with means for reducing asymmetricaleddy currents in the vacuum vessels and their coupling conduits and/orin the heat shields and their coupling conduits as describedhereinafter. Embodiments of means for reducing asymmetrical eddycurrents will now be explained.

FIG. 4 is a perspective view of the lower portion of a vacuum vessel 9of the first embodiment, and FIG. 5 is a perspective view of the lowerportion of the interior of the vacuum vessel 9. In this embodiment, thevacuum vessels 9 and coupling conduits 24 are made of a highelectric-resistance material such as stainless steel so that a magneticflux of the gradient magnetic field coil reaches the second heat shields8. In other words, eddy currents are deliberately generated in thesecond heat shields 8 and coupling conduits 23 so as to suppressgeneration of eddy currents in the vacuum vessels 9 and couplingconduits 24.

However, in a case of only making the vacuum vessels 9 and couplingconduit 24 of a high electric-resistance material, eddy currents aregenerated in the second heat shields 8 and coupling conduits 23asymmetrically with respect to the X-axis in the figure due topulse-like change of the magnetic fields generated by the gradientmagnetic field coils as explained for the vacuum vessels 9 shown in FIG.9. These asymmetrical eddy currents degrade the produced images.

Accordingly, in this embodiment, a slit (cut portion) 41 is furtherprovided in the vertical direction in each coupling conduit 22connecting the upper and lower first heat shields 7 and in each couplingconduit 23 connecting the upper and lower first heat shields 8. Theslits function as gaps in the circumferential direction of the couplingconduits 22, 23 to interrupt eddy current flow and reduce the asymmetryof the eddy currents. Since the density of eddy currents increases withincreasing proximity to the measurement space 5, the slits arepreferably provided nearest to the measurement space 5.

While the slits 41 formed continuously in the vertical direction in thisembodiment, each can be formed intermittently at several locations inthe vertical direction as shown FIG. 6. Although continuous slits asshown in FIG. 4 are preferable in view of prevention of eddy currents,intermittent slits offer better strength and resistance to deformationof the first and second heat shields 7, 8. In this case, it ispreferable that the slits 42 begin from at least the part of thecoupling conduits 22, 23 that are connected to the heat shields so thateddy currents having the largest effect can be prevented.

Although the coupling conduits 22, 23 of the heat shields both haveslits 41(42) in this embodiment, the slits 41(42) can, depending therequired performance, be made in only the coupling conduits 23 of thesecond heat shields 8. The slits 41 (42) can have a shape other than thethin and long shape shown in the drawings. In addition, the slits 41(42)can be covered with an electrically insulating lid using an adhesive ofhigh electric-resistance and high heat-conductivity in order to minimizedeterioration of the strength of the coupling conduits due to the slits.

Another embodiment of the present invention will be explained next.

This embodiment solves the problem of asymmetrical eddy currents causedby the presence of holes that connect the space inside the couplingconduits with the space surrounded by the heat shield, i.e., the holesat the ends (top and bottom) of the coupling conduits in thesuperconductive magnet device of FIG. 3.

In this embodiment, similarly to the aforementioned embodiment, thevacuum vessels 9 and the coupling conduits 24 are made of a highelectric-resistance material. In addition, as shown in FIG. 7, holes 40having the same shape as those of the holes at the ends of the twocoupling conduits 23 are formed symmetrically to the holes at the endsof the coupling conduits 23 with respect to the X-axis (the center axisin the anterior and posterior direction) and the Y-axis (the center axisin the transverse direction) on the second heat shield 8. Provision ofsuch holes reduces the asymmetry of the eddy currents generated in thesurface of the second heat shield 8.

As shown in FIG. 8, it is preferable in this embodiment for the holes 40to be covered with aluminum lids 43 adhered to the second heat shield 8using adhesive 44 of high electric-resistance and high heat-conductivityin order to reduce deterioration of heat shielding property owing toformation of the holes 40 in the second heat shield 8.

Although the drawings show the holes 40 formed in the second heat shield8, the first heat shield 7 can also be provided with holes and lids.

Similarly to the embodiment of FIG. 4, the first and second heat shieldsmay preferably have a continuous or intermittent slit in the verticaldirection in this embodiment.

There has been explained hitherto, first and second embodiments of thepresent invention that employ vacuum vessels and vacuum vessel couplingconduits made of a high electric-resistance material and cope withasymmetrical eddy currents by means of heat shields and heat shieldcoupling conduits. These embodiments are effective from the viewpoint ofmaintaining a high degree of vacuum. The present invention also providesan alternative embodiment, which copes with the asymmetrical eddycurrents by means of the vacuum vessels 9 and the coupling conduits 24.

Specifically, a third embodiment of the present invention employs thecoupling conduits 24 of the vacuum vessels 9 having slits similar tothose shown in FIGS. 5 and 6 and lids that cover the slits and areelectrically insulated from other parts, while it employs the vacuumvessels 9 and coupling conduits 24 made of a low electric-resistancematerial. Such a slit establishes a region where current does not flow.Alternatively, it is possible to cope with the asymmetrical eddycurrents by making only the coupling conduits 24 (but not the vacuumvessels 9) of a high electric-resistance material.

In a fourth embodiment, holes having the same shape as those of theholes connecting the inner space of the coupling conduit 24 to the innerspace of the vacuum vessels 9 are further provided symmetrically withrespect to both of the X-axis and Y-axis in order to further reduce theasymmetry of eddy currents. Lids of a high electric-resistance materialare also provided to keep the vacuum vessels 9 sealed. In the fourthembodiment too, the coupling conduits 24 of the vacuum vessels 9preferably have slits and slit lids.

It was explained in the foregoing that the superconductive magnet deviceof the present invention is applied to the apparatus shown in FIGS. 1–3.The magnet device of the present invention may be also applied to astatic magnetic field generating magnet device of an active-shieldingtype having no iron plate 33, a static magnetic field generating magnetdevice equipped with pole pieces on the opposing surfaces and so forth.Further, the number and configuration of the coupling conduits 24 may bechanged arbitrarily.

An overall configuration of an MRI apparatus which employs the magnetdevice of the present invention is shown in FIG. 10. This MRI apparatuscomprises a magnet 102 for generating a static magnetic field in a spacewhere a subject (patient) 101 is placed, gradient magnetic field coils103 for generating gradient magnetic fields in the space, an RF coil 104for generating a high frequency magnetic field in a measurement regionof the subject, and an RF probe 105 for detecting NMR signals emittedfrom the subject 101. The apparatus further comprises a bed 112 forconveying the subject into the static magnetic field space. The magnet102 is an open-type superconductive magnet device having an appearanceas shown in FIG. 1, in which generation of asymmetrical eddy currents issuppressed.

The gradient magnetic field coils 103 includes gradient magnetic fieldcoils for three directions X, Y, Z, each of which generates a gradientmagnetic field in accordance with signals supplied by a gradientmagnetic field power source 109. The RF coil 104 generates ahigh-frequency magnetic field in accordance with signals from an RFtransmitting unit 110.

Signals from the RF probe 105 are detected by a signal detecting unit106, processed by a signal processor 107 and further transformed toimage signals by calculation. Images are displayed on a display 108.

The gradient magnetic field power source 109, RF transmitting unit 110and signal detecting unit 106 are controlled by a controller 111according to a pulse sequence determined to correspond to themeasurement method and defines the timing of gradient magnetic field, RFmagnetic field application and signal detection.

In such an MRI apparatus, when the gradient magnetic field coils areoperated, eddy currents are generated within the vacuum vessels of themagnet 102 and coupling conduits thereof, and in the heat shields andthe coupling conduits thereof. However, since the apparatus employs themagnet device capable of preventing the generation of asymmetrical eddycurrents, artifacts caused by the asymmetrical eddy currents aresuppressed and images of good quality can be produced.

The present invention has been explained with reference to asuperconductive magnet device. The present invention can be applied tonot only an MRI apparatus having a superconductive magnet but also amagnet device using a permanent magnet and an MRI apparatus using thesame. In this case, columns for supporting a pair of magnets are madehollow and provided with slit portions to suppress the eddy currents.The slit portions may be continuous or intermittent in the longitudinaldirection of the columns and may be covered with a highelectric-resistance material to reinforce the strength of the supportingcolumns. In addition, when the material of the cover for covering themagnet is of low electric-resistance, recessed portions may be formed inthe positions on the cover corresponding to holes to which the columnsare attached so that asymmetry of the eddy currents generated in thecover can be cancelled.

1. An open magnet device comprising: A pair of vacuum vessels disposedacross a space in a vertical direction, wherein an object of inspectionis insertable within the space between the pair of vacuum vessels, atleast one cylindrical vacuum vessel coupling means for verticallyconnecting the inner spaces of the pair of vacuum vessels, and a pair ofmagnetic field generating means, one of the pair of the magnetic fieldgenerating means being disposed in one of the pair of vacuum vessels andanother of the pair of magnetic field generating means being disposed inanother of the pair of vacuum vessels, the pair of magnetic fieldgenerating means including a material having a superconductivity,wherein the pair of vacuum vessels and the vacuum vessel coupling meansare made of a material having a high electric resistance.
 2. The openmagnet device of claim 1, further comprising: at least one pair of heatshields disposed inside the pair of vacuum vessels and cylindrical heatshield coupling means for vertically connecting the heat shields throughspaces surrounded by the vacuum vessel coupling means, wherein the heatshield coupling means has at least one slit having a highelectric-resistance property and extending continuously orintermittently in the vertical direction.
 3. The open magnet device ofclaim 1, further comprising: at least one pair of heat shields disposedinside the pair of vacuum vessels and cylindrical heat shield couplingmeans for vertically connecting the heat shields through spacessurrounded by the vacuum vessel coupling means, wherein the heat shieldshave first holes opening into the inside of the heat shield couplingmeans and at least one of second holes and recesses having the sameshape as those of the first holes in positions symmetrical to the firstholes.
 4. The open magnet device according to claim 1, wherein the atleast one vacuum vessel coupling means have slit portions extendingcontinuously or intermittently in the vertical direction and membersconfigured for covering the slit portions which maintain the vacuumvessels in a vacuum state.
 5. The open magnet device according to claim1, wherein the vacuum vessels have first holes opening into the insideof the vacuum vessel coupling means and at least one of second holes andrecesses having the same shape as those of the first holes in positionssymmetrical to the first holes, the second holes of the vacuum vesselsbeing covered with a high electric resistance material.
 6. The openmagnet device of claim 1 further comprising: at least one pair of heatshields disposed inside the pair of vacuum vessels and cylindrical heatshield coupling means configured for vertically connecting the heatshields through spaces surrounded by the vacuum vessel coupling means,wherein the heat shield coupling means have slit portions exhibiting ahigh electric resistance and extending continuously or intermittently inthe vertical direction, and the heat shields have first holes openinginto the inside of the heat shield coupling means and at least one ofsecond holes and recesses having the same shape as those of the firstholes in positions symmetrical to the first holes.
 7. A magneticresonance imaging apparatus configured for using an open magnet deviceaccording to any one of claims 1–6.
 8. The open magnet device accordingto claim 1, configured for a magnetic resonance imaging apparatus,wherein the pair of magnetic field generating means include a pair ofmagnets disposed across the space in the vertical direction whichgenerates a static magnetic field, and supporting columns which connectthe magnets, wherein the supporting columns have slit portions extendingcontinuously or intermittently in the vertical direction.
 9. The openmagnet device configured for a magnetic resonance imaging apparatus ofclaim 8, wherein the supporting columns are hollow.
 10. The open magnetdevice configured for a magnetic resonance imaging apparatus of claim 9,further comprising: covers configured for the magnets, wherein thecovers have holes opening into the inside of the columns and recesseshaving the same shape as those of the holes in positions symmetrical tothe holes.
 11. A magnetic resonance imaging apparatus including an openmagnet device according to any one of claims 8–10.
 12. The open magnetdevice according to any one of claims 3 and 6, wherein the second holesof the heat shields are covered with a high heat-conductive material viaa material having high electric resistance and high heat-conductivity.13. The open magnet device according to claim 2, wherein the slits arearranged at a position, which is nearest to a measurement space of theopen magnet device.
 14. The open magnet device according to any one ofclaims 4, 6 and 8, wherein the slit portions are arranged at a positionwhich is nearest to a measurement space of the open magnet device. 15.The open magnet device according to claim 2, wherein the slits extendfrom at least a pad of the cylindrical heat shield coupling means at theconnection thereof to the heat shields of the open magnet device. 16.The open magnet device according to claim 4, wherein the slit portionsextend from at least a part of the cylindrical vacuum vessel couplingmeans at a connection thereof to the heat shields of the open magnetdevice.
 17. The open magnet device according to claim 6, wherein theslit portions extend from at least a part of the cylindrical heat shieldcoupling means at a connection thereof to the heat shields of the openmagnet device.
 18. The open magnet device according to claim 8, whereinthe slit portions extend from at least a part of the supporting columnsat a connection thereof to the heat shields of the open magnet device.19. The open magnet device of claim 1, further comprising: a pair ofheat shields disposed inside the pair of the vacuum vessels, a pair ofhelium containers disposed inside the pair of the heat shields, at leastone heat shield coupling conduit connecting the pair of heat shields,and at least one helium container coupling conduit disposed inside theheat shield coupling conduit and connecting the pair of heliumcontainers.
 20. The open magnet device of claim 1, wherein the vacuumvessel coupling means is disposed at one side of the space with respectto a plane including a central axis of a magnetic field extending in thevertical direction.
 21. The open magnet device of claim 1, wherein a topand a bottom of the open magnet device are surrounded by an upper ironplate and lower iron plate, and the upper and lower iron plates arecoupled magnetically by at least one iron column.
 22. The open magnetdevice of claim 1 wherein the open magnet device is an active-shieldingtype open magnet device.
 23. The open magnet device of claim 2, whereinthe at least one pair of heat shields comprise a plurality of pairs ofheat shields and a heat shield coupling conduit connecting the pair, andat least one of the heat shield coupling conduits has a slit.